1. Field of the Invention
This invention pertains generally to the field of navigation systems such as, but not limited to, aircraft navigation systems.
2. Description of the Related Art
The continuing growth of aviation has placed increasing demands on airspace capacity and emphasizes the need for the best use of the available airspace. These factors, along with the accuracy of modern aviation navigation systems and the requirement for increased operational efficiency in terms of direct routings and track-keeping accuracy, have resulted in the concept of “Required Navigation Performance” (“RNP”) standards—statements of the navigation performance accuracy necessary for operation within a defined airspace. Some of these standards appear in an Advisory Circular (“AC”) published by the United States (“U.S.”) Federal Aviation Administration (“FAA”) and in a Document (“DO”) published by the Radio Technical Commission for Aeronautics (“RTCA”). For example, the FAA has published AC 120-29A entitled “Criteria for Approval of Category I and Category II Weather Minima for Approach,” and the RCTA has published DO-236B entitled “Minimum Aviation System Performance Standards: Required Navigation Performance for Area Navigation.”
A low-visibility instrument approach procedure (“IAP”) that permits a pilot to operate to a lower altitude than permitted by a standard Instrument Landing Systems (“ILS”) (which is generally 200 feet height above threshold (“HATh”)) without visually identifying the runway environment may require the use of a radio altimeter system to define a missed approach point represented to the pilot as a decision height (“DH”).
In determining the DH, an aviation-governing authority such as the FAA surveys the approach corridor to the runway to determine the DH relative to the runway. In order to achieve lower minima (i.e., lower DH) for IAPs based upon a satellite navigation system such as the Global Positioning System (“GPS”), preparations have begun in the U.S. to implement the Next Generation Air Transport System (“NextGen”), a system designed to reduce the stress currently experienced by the U.S and address the expected growth in aircraft operations forecasted through 2025. At the time of this writing, the lowest minima for GPS-based approaches such as the Localizer Performance with Vertical Guidance (“LPV”) include a DH of 200 feet HATh. To achieve a lower DH, surveys of approach corridors will mean a survey more than 2,700 GPS IAPs, the amount of such procedures available in the U.S.; moreover, in the next five years, there is a potential of more than 5,000 such procedures, many of which could include the use of a radar altimeter system(s) when determining the DH. In addition, the height value of the DH may differ between airports where the relative terrain in the approach corridor is unique.
The radar altimeter system produces radar altimeter readings that are independent of aircraft decent rate. Because of “noise” produced in a radar altimeter system and because of variations in terrain elevations, the height above terrain measured by the radar altimeter system may change quickly. Also, the inclusion of the radar altimeter system in an IAP may require the aviation-governing authority to perform routine ground maintenance in the approach corridor so that obstacles such as trees and towers do not present false readings. A significant amount of the pilot's attention may be needed when watching for the arrival at the decision height when such conditions exist.
RNP may include performance and functional requirements indicated by the RNP type. The RNP type defines the total system error (“TSE”) that may be allowed in lateral and longitudinal dimensions within a particular airspace, where the TSE takes into account path definition errors (“PDE”), navigation system errors (“NSE”), and flight technical errors (“FTE”). The RNP type is used to specify navigation requirements for the airspace or a navigation system that provides a specified level of accuracy defined by a lateral area of confined airspace in which an RNP-certified aircraft operates. For example, an RNP 0.3 level requires a normal navigational accuracy of 0.3 nautical miles (“NM”) at least 95 percent of the total flight time; similarly, an RNP 0.1 level requires a normal navigational accuracy of 0.1 NM at least 95 percent of the total flight time. Hence, a lower level of RNP means a greater level of navigational accuracy. With the availability of GPS, future airspace systems may rely heavily on GPS-based navigation to which stringent standards for NSE may be applied. An IAP specifying an RNP level of less than RNP 0.3 or lower than the standard minima at airports not equipped with Instrument Landing Systems (“ILS”) airports may require a method to detect GPS position errors before the 6.2 second time-to-alert standard that currently exists. A similar requirement may be made for manufacturers and/or end-users wishing to take advantage of the standards published in RTCA DO-315B entitled “Minimum Aviation System Performance Standards (MASPS) for Enhanced Vision Systems, Synthetic Vision Systems, Combined Vision Systems and Enhanced Flight Vision Systems.”
One way to achieve a greater level of navigational accuracy is to reduce the TSE. Because the TSE may be comprised of the PDE, NSE, and FTE, a reduction of any of these—such as the NSE—could reduce the TSE which, in turn, could produce a greater level of navigational accuracy. A reduction of NSE could be obtained by monitoring and/or augmenting navigation data provided by one or more navigation systems with an independent source of navigation data. Such independent source could be used to generate altitude and/or height data.
The independent source could also be used to validate the navigation system navigation data by confirming the navigation system navigation data falls within lower desired tolerances commensurate with a lower NSE. The use of an independent source to validate navigation system navigation data is disclosed by Bell et al in a separate an application entitled “Navigation Data Validation Systems and Methods,” which is hereby incorporated by reference in its entirety.